Recombinant Hormones Make Mares Cycle in Winter (AAEP 2011)

People have successfully shortened or lengthened mares’ estrous cycles for decades, but it’s a much taller order to induce cycling in a mare that’s not cycling at all. However, researchers recently reported good success with inducing ovulation in winter anestrous (noncycling) mares via a new hormone therapy regimen at the 2011 American Association of Equine Practitioners convention held Nov. 18-22 in San Antonio, Texas.

Veterinarians and owners have traditionally used artificial lights to stimulate mares to ovulate earlier in year (February) instead of waiting until the onset of the natural or physiologic breeding season (April). People have also successfully shortened or lengthened estrous cycles to synchronize donor and recipient mares for oocyte or embryo transfer.

Researchers at Colorado State University, the University of Kentucky, and the University of California, Davis, have taken this process one step further by using recombinant equine follicle-stimulating hormone (reFSH) to induce follicle development and subsequent ovulation in deep anestrus mares (the stage they’re generally in midwinter).

The hormone eFSH has been used in the past to stimulate donor mares' ovaries to increase the number of ovulations (i.e., superovulation) and thereby increase the number of embryos that may be recovered during one cycle.

"Recombinant equine FSH is a synthetic version of equine FSH in that it is produced in a laboratory," explained presenter Pat McCue, DVM, PhD, Dipl. ACT, professor in the Department of Clinical Sciences at Colorado State.. " ‘Regular’ equine FSH is harvested from the pituitary glands of mares and is no longer available."

At all three universities, a treatment group of mares received reFSH injections twice daily for 15 days or until researchers detected an ovarian follicle at least 35 mm wide. All treated mares had significant follicular development (35 mm+ follicles), and 76.7% had an average of 3.8 ovulations after injection with human chorionic gonadotropin (hCG, given after follicles at least 35 mm in diameter were detected, to cause the follicles to release the eggs). No control mares (those receiving a placebo treatment) exhibited follicular development or ovulated during the observation period.

Following ovulation, all treated mares returned to anestrus (not cycling) until spring, at around the same times as untreated control mares.

“Deep anestrus is the most challenging case (for inducing ovulation),” McCue commented. “I was surprised they all responded, but not that they went back into anestrus.”

McCue listed the following situations in which this management strategy could be useful:

Managing anestrus/transitional mares that are not under lights, not responding to lights, or in conjunction with lights;

Managing mares without significant follicular development during the breeding season;

Treating problem acyclic mares, such as those in postpartum anestrus or those that are acyclic during the normal physiologic breeding season; and/or

Causing superovulation (release of multiple eggs per ovulation) in cycling mares to increase embryo recovery rates or increase the probability of a pregnancy in mares carrying their own foal.

Recombinant equine FSH is not yet currently available on the market, but the company that provided the product for this study hopes to have FDA approval for it in the future.